Process for desulphurizing a hydrocarbon cut in a simulated moving bed

ABSTRACT

The invention describes a process for desulphurizing a gas oil type hydrocarbon cut by adsorption to obtain a desulphurized effluent containing less than 10 ppm by weight in a yield of more than 95% by weight. Said process comprises a step for simulated moving bed adsorption of sulphur-containing compounds in the feed, a raffinate distillation step and an extract distillation step.

CLAIM OF PRIORITY

Priority is claimed of French application 04/12.415 filed Nov. 23, 2004.

FIELD OF THE INVENTION

The present invention relates to a process for desulphurizing a distillate type hydrocarbon cut by simulated moving bed adsorption. The term “distillate type cut” means a cut from distilling crude or from a conversion unit such as catalytic cracking, with a distillation range in the range 150° C. to 450° C.

We shall henceforth term this cut a “gas oil” cut, but such a designation is not restricting in nature. Any hydrocarbon cut containing sulphur and with a distillation range similar to that of a gas oil cut may be used in the process of the present invention.

The process of the invention can thus be used to produce a desulphurized cut with 10 ppm by weight of sulphur (S) or less, or even 5 ppm by weight of sulphur, or even less than 1 ppm by weight of sulphur, starting from a feed to be treated with a sulphur content which may be several tens of ppm by weight up to 2% or even 3% by weight. The notation “ppm by weight” means parts per million by weight and is equivalent to 10⁻⁶ kg/kg.

Further, the gas oil yield of said process is substantially higher than that of a fixed bed process.

Future specifications regarding automobile fuels envisage a large reduction in sulphur content in fuels, in particular gas oils. Said reduction is intended to limit the amount of oxide of sulphur and nitrogen in automobile exhaust gases. European legislation prescribes gas oil fuel specifications; in 2000, they were 350 ppm by weight of sulphur, and in 2005 will drop to 50 ppm by weight of sulphur; in 2009, they fall to 10 ppm by weight of sulphur.

Changes in specifications for the sulphur content of fuels thus either require improving existing hydrotreatment catalytic processes, resulting in a substantial overconsumption of hydrogen and/or an increase in operating pressure, or the development of novel deep desulphurization processes for gas oils, or a combination of the two.

EXAMINATION OF PRIOR ART

Gas oil desulphurization methods include processes for purification by adsorption of sulphur-containing compounds on a selective adsorbant and provide an advantageous alternative to conventional hydrodesulphurization processes.

As an example, United States patent U.S. Pat. No. 4,337,156 (UOP 1982) recommends the use of a KX type zeolite and a 1-octanol type desorbant to separate polar compounds (sulphur-containing, nitrogen-containing, oxygen-containing) by simulated moving bed (SMB) separation of a naptha cut (term used by the skilled person to designate a gasoline cut with an initial boiling point of about 70° C. and an end point of about 220° C.).

The sulphur-containing compounds present in the naphtha cut are of the thiophene type and not of the benzo or dibenzothiophene type as they constitute the sulphur-containing compounds which are the most difficult to eliminate in the case of a distillate, i.e. those which are encountered in the context of the present invention. U.S. Pat. No. 5,454,933 describes a gas oil desulphurization process which consists of linking a conventional hydrotreatment to eliminate sulphur-containing compounds, known as an “easy sulphur” process, with a process for adsorbing difficult sulphur-containing compounds over an activated carbon with a specific surface area in the range 800 to 1200 m²/g and having a specific pore structure. Said sulphur-containing compounds which are difficult to eliminate, termed “hard sulphur” compounds, correspond to beta-substituted dibenzothiophene type aromatics.

The adsorption process described in that patent is not designed to treat the entire starting gas oil feed, but necessitates prior hydrotreatment.

French patent application FR-A-02/03314 proposes a process for desulphurizing a hydrocarbon feed on a complexing solid based on π electron acceptors. Said process may be preceded by a fractionation column which can produce a light effluent to specifications and a heavy effluent which has to be desulphurized. In the case of fixed bed operation, the losses of product to be desulphurized are not negligible due to physical filling of the pore volume of the adsorbent. That patent proposes a remedy by employing different washing fluids having variable adsorption forces that can thus reduce the hydrocarbon loss without, however, avoiding them entirely.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flowchart of the process of the invention in its most general scope.

FIG. 2 shows a variation of the flowchart of the process according to the invention.

BRIEF DESCRIPTION OF THE INVENTION

The invention can be defined as a process for deep desulphurization of a gas oil type hydrocarbon feed with a distillation range in the range 150° C. to 450° C. which may contain up to 3% of sulphur, the process comprising the following steps in succession:

-   -   a step for adsorbing sulphur-containing compounds from the feed         to be treated carried out in at least one first adsorption         column functioning as a simulated moving bed (SMB) and         containing a plurality of beds constituted by an adsorbent solid         having a selectivity which is different for sulphur-containing         hydrocarbons and non sulphur-containing hydrocarbons, said         column comprising at least four functional zones;     -   a first zone between the point for introducing desorbant and the         point for withdrawing extract; a second zone between the point         for withdrawing extract and the point for introducing feed; a         third zone between the point for introducing feed and the point         for withdrawing raffinate; and a fourth zone between the point         for withdrawing raffinate and the point for introducing         desorbant;     -   a step for distilling raffinate carried out in at least one         distillation column, from which a gas oil effluent is extracted         as well as a stream of desorbant at least part of which is         recycled to the first adsorption column;     -   a step for distilling extract carried out in at least one         distillation column, from which an effluent containing         sulphur-containing impurities is withdrawn along with a         practically pure desorbant stream at least part of which is         recycled to the first adsorption column.

The sulphur content in the gas oil obtained is generally less than 10 ppm by weight, preferably less than 5 ppm by weight, and more preferably less than 1 ppm by weight, with a yield with respect to the inlet feed which is generally more than 97% by weight and preferably more than 99%.

In a variation of the process, the feed to be treated may be sent in advance to a distillation column located upstream of the simulated moving bed adsorption unit, from which an overhead stream is withdrawn at least part of which is used as a desorbant, and a bottom stream is withdrawn which is introduced as the feed to the simulated moving bed adsorption unit.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be better understood from the flowchart of the process represented in FIG. 1.

The feed (1) enters the adsorption desulphurization unit operating as a simulated moving bed (2). This adsorption desulphurization unit is constituted by at least one adsorption column containing a plurality of interconnected adsorbent beds having a selectivity in favour of sulphur-containing compounds over the chemical families of the feed (alkanes and aromatics) which are to be purified.

Said adsorption column comprises at least four zones delimited by injections for a mixture (1) constituting the adsorption and desorbant feed (9 b), and by withdrawals for a raffinate (3) containing the desulphurized gas oil mixed with desorbant, and an extract (4) mainly containing sulphur-containing compounds eliminated as a mixture with desorbant.

Zone 1 for desorption of sulphur-containing compounds is included between the desorbant injection (9 b) and the extract withdrawal (4).

Zone 2 for desorbing alkanes and aromatics is between the extract withdrawal (4) and the adsorption feed injection (1).

Zone 3 for adsorbing sulphur-containing compounds is between the feed injection (1) and the raffinate withdrawal (3).

Zone 4 is between the raffinate withdrawal (3) and the desorbant injection (9 b) and can adsorb the alkanes and aromatics.

The step for separating streams (3) and (4) is carried out by means of two distillation columns (5) and (6) respectively supplied with the raffinate (3) and the extract (4) which can eliminate substantially all of the desorbant at the bottom of the column, for example.

A stream (8) of desulphurized gas oil containing less than 10 ppm by weight of sulphur, preferably less than 5 ppm by weight of sulphur and more preferably less than 1 ppm by weight of sulphur, is withdrawn from the head of the column (5), and a mixture (10) of sulphur-containing and nitrogen-containing compounds is withdrawn from the head of column (6).

This mixture may itself advantageously be mixed with a refinery stream with suitable sulphur specifications, with boiling points which are compatible with those of the mixture produced, such as a fuel oil, for example.

This mixture may also be recycled to a conventional hydrotreatment unit which can eliminate the recycled sulphur-containing compounds by increasing the sulphur content of the feed, as the catalytic activity of the hydrodesulphurization process is directly linked to the inlet concentration of sulphur to be treated.

Desorbants (9) and (11) are recovered from the bottom of columns (5) and (6) to form the stream (9 a) which is returned to a simulated moving bed (SMB) desulphurization unit (2) with optional makeup of desorbant (12) corresponding to any losses of desorbant suffered in the distillation columns (5) and (6).

The various streams (9), (11) and (12) form the addition of desorbant (9 b) which is introduced into the simulated moving bed column (2).

The stream (7) constitutes the recirculation stream which is indispensable to the operation of a simulated tnoving bed column. It is constituted by regeneration solvent and gas oil in proportions which vary with time.

FIG. 2 describes a variation in the invention in which a distillation step corresponding to the unit (1 e) is placed upstream of the flowchart described in FIG. 1.

This distillation step consists of sending the feed (1) to a distillation tower (1 e) which produced a heavy sulphur-containing hydrocarbon cut (1 b) which is sent to the SMB adsorption unit (2) as described in FIG. 1, and a light hydrocarbon cut (1 a) the sulphur content of which is equal to or below the required specifications.

Said desulphurized cut (1 a) may also be used as a desorption agent in the SMB adsorption unit (2), in which case, once a pseudo steady state has been reached, the portion (1 d) of the stream (1 a) is used as a makeup desorbant and mixed with the stream (9 a) to form the stream (9 b) of desorbant of the adsorption column 2. The remaining portion (1 c) of desulphurized light hydrocarbons has already satisfied the required specifications and could thus act as a base in the commercial gas oil

In a variation of the process, it is possible to process a gas oil which will already have been desulphurized in a conventional catalytic hydrodesulphurization unit which is well known to the skilled person.

The sulphur content of the gas oil produced by said unit will vary depending on the hydrotreatment operating conditions. The gas oil produced, the sulphur content of which may be between 10 ppm by weight and 1000 ppm by weight, could be treated using one or other of the variations of the process of the invention corresponding to FIGS. 1 and 2.

The adsorbent used in a SMB adsorption unit is generally selected from the following classes of conventional adsorbents: activated charcoal, zeolites, silicas, aluminas, silica-aluminas, used catalysts, resins, clays, pillaxed clays, reduced metals or oxides and any possible mixture between these different families of adsorbents.

In accordance with one characteristic of the process, the adsorbent used in the SMB adsorption unit is selected from the activated charcoal class, as said solids have sufficient selectivity between the sulphur-containing molecules and the remainder of the gas oil matrix. Preferred activated charcoal types are those which have a specific surface area of more than 1200 m²/gram and a total pore volume of more than 0.5 cm³/gram, the precursor possibly being of any type, and the type of activation used to create the porosity possibly being either physical, or chemical, or a combination of the two.

Whatever the adsorbent solid used in the process, the total pore volume is preferably 0.5 cm³/gram or more, and the fraction of pore volume included in the microporosity of said solid adsorbent is preferably 0.2 cm³/gram or more.

The microporosity is defined as the category of pores with a diameter of less than 20 angstroms (2 nanometers, i.e. 2×10⁻⁹ metres).

The number of adsorbent beds constituting the simulated moving bed adsorption unit is generally less than 24, and preferably less than 15.

The desorbant may be selected from the following chemical classes: nitrogen-containing compounds, alcohols, ethers, aromatics, desulphurized light cuts, or any other refinery stream or mixture thereof. As an example, aromatics may preferably be selected.

The ratio of the desorbant to the feed in the simulated moving bed separation unit is generally in the range 0.5 to 2.5 by volume, preferably in the range 0.7 to 2.0.

The operating temperature may be between ambient temperature and the end point of the hydrocarbon cut to be treated, knowing that liquid phase operation is required. In general, a temperature of 50° C. to 350° C. is used, preferably in the range 50° C. to 250° C.

The operating pressure may be between the bubble point of the lightest compound and 15 bars absolute (1.5 MPa), to guarantee the existence of a liquid phase throughout the SMB adsorption unit, knowing that the performance of the process is less dependent on this parameter. However, it may have an influence on the equipment cost.

Examples of the Invention

Consider an activated charcoal with a specific surface area of 1440 m²/g with a total pore volume of 1.7 cm³/g, with a microporosity pore volume fraction of 0.35 cm³/gram and such that the elution order of the different families of the distillate is as follows:

alkanes;

aromatics: mono-, di- and tri-;

easy sulphur (sulphur-containing compounds which are easy to eliminate);

hard sulphur (sulphur-containing compounds which are difficult to eliminate);

nitrogen-containing compounds.

Example of Processing a Low Sulphur Gas Oil (50 ppm of Sulphur)

A gas oil containing 50 ppm by weight of sulphur was purified in a simulated moving bed on a pilot unit comprising 15 beds of 609.6 cm³ each divided into 4 zones in the following configuration: zone 1=3 beds; zone 2=5 beds; zone 3=5 beds; and zone 4=2 beds.

The operating conditions were as follows:

Temperature: 210° C.

Pressure: such that liquid was present at all points in the circuit, namely 3 bars absolute (0.3 MPa);

Injection, withdrawal and recycle flow rates and operating conditions:

Feed: 200.0 cm³/min;

Solvent: 147.0 cm³/min of toluene;

Extract: 57.0 cm³/min

Raffinate: 290.0 cm³/min

Recycle flow rate (in zone 1): 193 cm³/min.

The valve permutation time (or period) was 152.0 seconds.

After distilling toluene, the raffinate obtained delivered a gas oil with 1.5 ppm by weight sulphur content in a purity of 99.5%. The productivity of the unit, expressed as the volume of gas oil produced per volume of adsorbent and per unit time, was 1.31 m³/m³.h).

Example of Processing a High Sulphur Content Gas Oil (1.5% Sulphur)

Gas oil containing 1.5% of sulphur (by weight) was purified in a simulated moving bed on a pilot unit comprising 15 beds of 609.6 cm³ each divided into 4 zones in the following configuration: zone 1=3 beds; zone 2=5 beds; zone 3=5 beds; and zone 4=2 beds.

The operating conditions were as follows:

Temperature: 210° C.

Pressure: such that liquid was present at all points in the circuit;

Injection, withdrawal and recycle flow rates and operating conditions:

Feed: 105.0 cm³/min;

Solvent: 147.0 cm³/min of toluene;

Extract: 82.0 cm³/min

Raffinate: 170.0 cm³/min

Recycle flow rate (in zone 1): 193 cm³/min.

The valve permutation time (or period) was 152.0 seconds.

After distilling toluene, the raffinate obtained delivered a gas oil with 3.5 ppm by weight of sulphur in a purity of 99.5%. The productivity of the unit, expressed as the volume of gas oil produced per volume of adsorbent and per unit time, was 0.67 m³/(m³.h).

The entire disclosures of all applications, patents and publications, cited herein and of corresponding French application No. 04/12.415, filed Nov. 23, 2004 are incorporated by reference herein.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. 

1. A process for deep desulphurization of a hydrocarbon feed with a distillation range in the range 150° C. to 450° C. containing up to 3% of sulphur, the process comprising the following steps in succession: a step for adsorbing sulphur-containing compounds from the feed (1) carried out in at least one first adsorption column (2) functioning as a simulated moving bed and containing a plurality of beds of an adsorbent having a selectivity which is different for sulphur-containing hydrocarbons and non sulphur-containing hydrocarbons, said column comprising at least four functional zones; a first zone between the point for introducing desorbant (9 b) and the point for withdrawing extract (4); a second zone between the point for withdrawing extract (4) and the point for introducing feed (1); a third zone between the point for introducing feed (1) and the point for withdrawing raffinate (3); and a fourth zone between the point for withdrawing raffmate (3) and the point for introducing desorbant (9 b), the ratio of the desorbant to the feed being in the range 0.5 to 2.5 by volume; a step for distilling raffinate (3) carried out in at least one distillation column (5), from which a desulphurized gas oil effluent (8) is extracted as well as a stream of desorbant (9) at least part of which is recycled to the first adsorption column (2); a step for distilling extract (4) carried out in at least one distillation column (6), from which an effluent (10) containing sulphur-containing impurities is withdrawn along with a practically pure stream (11) of desorbant at least part of which is recycled to the first adsorption column.
 2. A deep desulphurization process according to claim 1, wherein the sulphur content in the desulphurized gas oil (8) is less than 10 ppm by weight, with a yield with respect to the inlet feed which is generally more than 97% by weight.
 3. A deep desulphurization process according to claim 1, wherein the solid adsorbent used in the adsorption unit is activated charcoal with a specific surface area of more than 1200 m²/g and a total pore volume of more than 0.5 cm³/gram.
 4. A deep desulphurization process according to claim 1, wherein the pore volume fraction in the microporosity of said adsorbent is more than 0.2 cm³/gram.
 5. A deep desulphurization process according to claim 1, wherein the operating temperature of the adsorbent unit is in the range 50° C. to 350° C.
 6. A deep desulphurization process according to claim 1, wherein the simulated moving bed adsorption unit comprises at most 24 beds, preferably at most 15 beds.
 7. A deep desulphurization process according to claim 1, wherein the simulated moving bed adsorption unit has a ratio of desorbant to feed in the range 0.7 to 2.0 by volume.
 8. A deep desulphurization process according to claim 1, wherein the feed to be treated is sent in advance to a distillation column located upstream of the simulated moving bed adsorption unit, from which an overhead stream (1 a) is withdrawn at least part of which is used as desorbant, and a bottom stream (1 b) is withdrawn which is introduced as the feed to the simulated moving bed adsorption unit (2).
 9. A deep desulphurization process according to claim 1, wherein the stream (10) of sulphur-containing impurities is sent as a mixture to a fuel oil cut (fuel).
 10. A deep desulphurization process according to claim 1, wherein the effluent containing sulphur-containing impurities (10) is recycled to a hydrodesulphurization unit.
 11. A deep desulphurization process according to claim 8, wherein the stream (10) of sulphur-containing impurities is sent as a mixture to a fuel oil cut (fuel).
 12. A deep desulphurization process according to claim 8, wherein the effluent containing sulphur-containing impurities (10) is recycled to a hydrodesulphurization unit.
 13. A process according to claim 2, wherein the sulphur content in the desulphurized gas oil is less than 5 ppm by weight.
 14. A process according to claim 2, wherein the sulphur content in the desulphurized gas oil is less than 1 ppm by weight.
 15. A process according to claim 2, wherein said yield is more than 99% by weight.
 16. A process according to claim 5, wherein said operating temperature is in the range of 50° to 250° C.
 17. A process according to claim 6, wherein the adsorption unit comprises at most 15 beds.
 18. A deep desulphurization process according to claim 2, wherein the solid adsorbent used in the adsorption unit is activated charcoal with a specific surface area of more than 1200 m²/g and a total pore volume of more than 0.5 cm³/gram.
 19. A deep desulphurization process according to claim 2, wherein the pore volume fraction in the microporosity of said adsorbent is more than 0.2 cm³/gram.
 20. A deep desulphurization process according to claim 3, wherein the simulated moving bed adsorption unit has a ratio of desorbant to feed in the range 0.7 to 2.0 by volume. 